I did send my 4 oldest children to pre-school. My purpose in sending them was to experience socialization with other kids, getting new experiences, and learning to listen to other adults besides my husband and me. We had an awesome preschool close to our home that was a farm school and they got to interact with the animals and take nature walks on a daily basis, weather permitting of course. They did no academic instruction, just fun activities, (art, pretend play, music, stories...) based on a theme in a learning center type environment. Just for fun. I did plenty of academic type activities at home and like you, my children had all the basics well before kindergarten. I just thought it was good for them, given that we had this amazing preschool so nearby, to get some time to do something else to stimulate them. And honestly, having 4 young children at the time, I was eager for some time to get a few things done for a couple of hours 2-3 days a week! I am a firm believer in the idea that kids are in traditional school from age 5 until who knows when, at least 13 years of education in an institutional setting, and probably many more than that! Why in the world do we as a society have this need to take these little tiny children and make them sit down and do formal academics in an institution any earlier than we absolutely have to! The learning they do from birth to kindergarten, should be through play and new experiences and interactions! Playing and pretend time is the best time for learning at this age.
I now have a 3 year old and we have since moved from the area where the farm school was. I am leaning toward skipping preschool and just doing the play group, sunday school, and music, and movement classes. I don't think he will miss out on anything doing this, but if I send him, I would miss out on all the time he is away from me and selfishly, I am treasuring this time, as he is likely our last child!
So I guess my 2 cents is that if you can give him interaction with other kids and adults as well as different experiences like field trips, play groups and different classes, and especially if you would have to go back to work to afford it, then I would skip the preschool experience. He is getting all he needs and more without it, in my opinion.
Even though society would like us to believe otherwise, I don't think there is any long term data that says that kids who go to preschool have any advantage over kids who don't and that over the long term, they do any better than their pre-schooled peers down the road.